In color ink jet printing, very small dots of colored ink are transmitted from the print head to the substrate (e.g., paper) by heating a portion of the ink which is in the proximity of a nozzle in the print head. Heating the ink will cause a bubble to form. The formation of the expanding bubble pushes a droplet of ink out through the nozzle, and thence to the substrate.
By printing patterns of these small dots of colored ink, halftone images can be formed. Conventionally, at least three different primary color inks are used. For example, many color ink jet printers use cyan, magenta and yellow as the three primary colors. By halftone printing a combination of the primary colors in a given area of the image, a multitude of different colors can be achieved by controlling the coverage of each of the primary colors in the given area. In order to control the coverage of each primary color, the number of dots of each primary color is controlled. Printing more dots of a primary color in the area will give greater coverage by that primary color. Printing fewer dots will give less coverage. In this way, full color halftone images are achieved with an ink jet printer.
Accordingly, the number of dots of each primary color within various portions of the image are controlled depending on the image data. The image data is stored in the form of pixels, where each pixel has color values for each primary color. For example, a pixel of image data may include a cyan value (0 to 255), a magenta value (0 to 255) and a yellow value (0 to 255). Now, a pixel corresponds to an area of the image so small that only a single dot of each primary color can be printed (or not printed) at the pixel location. Therefore, a halftone algorithm is used to convert the color value for the pixel into a determination of whether or not a dot of the particular primary color will actually be printed at the pixel location.
Conventional halftone algorithms are mathematically and/or statistically complex. Some conventional halftone algorithms include Bayer, Floyd-Steinberg, Blue Noise Mask. Detailed explanation of various conventional halftone algorithms is not necessary here. However, it is noted that although the color values for a pixel have a great influence over whether a dot of each primary color will actually be printed at that pixel location, other factors may also be involved, such as color values of pixels elsewhere in the image data or a pixel-by-pixel comparison with a masking matrix, such as a Bayer matrix. For example, U.S. Pat. No. 5,708,518 (the entire specification of which is herein incorporated by reference) to Parker et al. discusses the use of such a masking matrix to determine whether a dot is printed at each pixel location in a halftone image.
Despite the complexity engendered by conventional halftone algorithms, some rough generalizations hold true. Specifically: (1) an area of the image data which is characterized by high color values for a particular primary will tend to have many dots of that primary color printed at corresponding pixel locations on the substrate, (2) an area of the image data which is characterized by low color values for a particular primary will tend to have relatively few dots of that primary color printed at corresponding pixel locations on the substrate, (3) areas which are characterized by intermediate color values or a mixture of high, low and intermediate color values will tend to have an intermediate number of dots.
The color values, used in conjunction with halftone algorithms, control the number of dots printed of each primary, and thereby indirectly control the coverage on the substrate of each primary color. This scheme works well to predictably reproduce colors as long as the coverage provided by each dot remains fairly predictable and constant. In order for dot coverage to remain fairly predictable and constant, the mass of ink in each droplet ejected by the print head must remain fairly predictable and constant. If the droplet mass varies, then coverage will vary, and the colors produced on the substrate will also vary.
One phenomenon which causes droplet mass to vary is temperature of the print head. Increased print head temperature affects the mechanics of ink flow and bubble formation such that larger droplets of ink are produced. Conversely, decreased temperature affects the mechanics of ink flow and bubble formation such that smaller droplets of ink are produced.
This temperature variation and consequent droplet mass variation is problematic for two reasons. First, if the print head temperature during actual printing is different than the print head temperature assumed in generating the color values of the image data and the halftone algorithm, then the colors of the printed image will not correspond well with the image data. Second, the print head temperature may vary during the actual printing process and cause variations within a single printed image. The variations within an image may be variations within each swath, and/or variations from between the swaths.
Various concepts have been proposed to grapple with the problem of print head temperature variation.
It has been proposed to slow the speed of the print head, lengthen the time interval (i.e., period) between which dots are ejected, and/or provide intermittent time intervals of rest for the print head during the printing process. The idea behind these proposals is that the print head will have time to cool and thereby counter heat buildup which can be caused by repeated firing. These proposals may indeed keep print head temperature relatively constant within a single image by countering heat buildup. However, printing is slowed, which is generally undesirable. Also, temperature must be kept within a range of about 5.degree. C. to prevent noticeable variations. This is quite difficult to do, especially when the print head may fire tens of thousands of times over the course of a single swath.
Thermostatically controlled heaters to maintain the print head at a relatively constant temperature have been proposed. This proposal requires additional heater and thermostat hardware, and may be difficult to implement in practice.
Another proposal involves control, based on print head temperature, of the firing energy which is supplied to the print head to print each dot. For example, U.S. Pat. No. 5,614,934 to Yoshida et al. discloses a sublimation type printer wherein the energy supplied to the sublimation type print head is controlled based upon temperature. While this may work well for sublimation type printers, it may be more difficult to implement this type of design in ink jet printers because the mechanics of bubble generation in a thin film of liquid ink make it difficult to precisely control droplet mass by controlling the firing energy.
In view of the difficulties and potential shortcomings in the various proposals, there is a need for an ink jet printer which accounts for temperature variations in the print head.